Heating Ventilation and Air Conditioning (HVAC) systems typically operate on 24V AC that is derived from a transformer connected to the AC power line of the home. This single source must provide power for all the components within the HVAC system. In newer systems, power is typically delivered via one pair of wires, while one or more additional wires are used to provide communication. System installation can become a serious issue if the existing house wiring does not support the wiring required for the desired features. It is therefore desirable to have a system that needs only 2 wires to provide both power and communication. This would allow for system component installation unconstrained by the amount of existing wiring.
Two wire solutions currently exist for providing power and communications. However, these power line carrier (PLC) solutions are geared toward communication over the AC power lines within a home where the loads need uninterrupted full cycle AC power. This requirement drives the nature of PLC devices. PLC devices typically communicate by inducing a high frequency voltage or current onto the AC power line and are relatively expensive.